1. Field of the Invention
This invention relates to a method and apparatus for concentration and recovery of halocarbons from process effluent gas streams. The present invention further relates to a process of removing by-products and impurities from halocarbon-containing waste streams to enhance such concentration and recovery process.
2. Description of the Related Art
Fluorinated and chlorinated compounds are used in semiconductor etch, chemical vapor deposition ("CVD"), and process tool cleaning processes. Examples of compounds which are widely used in such processes include perfluorocarbons ("PFCs"), fluorinated hydrocarbons, and chlorofluorocarbons as well as sulfur hexafluoride and nitrogen trifluoride, all of which will be collectively referred to herein as "halocarbon." Specific examples include C.sub.2 F.sub.6, CF.sub.4, CHF.sub.3 and SF.sub.6. These compounds are suspected of causing global warming by what has come to be known as a "greenhouse" effect. While long-term effects are unknown, current data suggest that compounds of this type are accumulating in increasing concentrations in the upper atmosphere, and that they can persist there for thousands of years. Manufacturers of many of these compounds, especially the perfluorocarbons are establishing policies that require customers to ensure that a high percentage of the PFCs are recycled or prevented form reaching the atmosphere. The U.S. Environmental Protection agency is requiring that PFC emission levels return to 1990 levels by the year 2000.
Incineration (combustion) has been shown to be an effective means of destroying halocarbons including PFCs. However, incineration requires considerable capital investment. In addition, because of the stability of PFCs, they must bc heated to over 1000.degree. C. before oxidation occurs. The reaction that takes place: EQU C.sub.2 F.sub.6 +2O.sub.2 +3H.sub.2 .fwdarw.2CO.sub.2 +6HF
produces toxic and corrosive hydrogen fluoride, which must then be neutralized. Typically, a wet scrubber would be used to neutralize the hydrogen fluoride, adding to the overall cost and complexity, and generating large volumes of aqueous waste whose disposal may be inconvenient. Also, burning processes that involve hydrogen raise safety concerns. If the halocarbon is present in the effluent stream in a dilute concentration, incineration is especially cost-ineffective. In general, in the effluent streams from semiconductor etch reactors, the halocarbon species are expected to be present in concentrations of only a few percent, e.g., 0.1-5%.
Halocarbon recovery and recycling is an approach potentially providing advantages to the environment as well as providing a simpler apparatus and methodology for keeping halocarbon emissions to a minimum. Recovery and recycling are relatively simple for the liquid halocarbons, which can be trapped by refrigeration units. For the gaseous halocarbons, this approach may be too energy-intensive. However, it has been found that because of their chemical inertness, the perfluorocarbons, sulfur hexafluoride (SF.sub.6), and the Freons (CF.sub.4, C.sub.2 F.sub.6, and CHF.sub.3) that are used in the semiconductor industry are well-suited to recovery by adsorptive processes.
It is known that halocarbons may be adsorbed by various sorbent materials, including porous carbons, zeolites, silicas and aluminas. Wood and Stampfer (Carbon 31, pp. 195-200, 1993) studied adsorption for fifteen fluorocarbons on beds of activated carbon such as are used for removing gases and vapors from air. Their data may be used to predict the performance of a sorbent bed for any of these fluorocarbons after the bed has been characterized for one of the compounds. Packed beds of such activated carbons are used in applications ranging from air sampling tubes, respirator cartridges, to large industrial effluent filters, to adsorb halocarbons. Beds of porous adsorbent materials have been reported to be useful for the separation and recovery of volatile fluorocarbons.
Izumi et al. (Japanese Patent Application 03/135,410, Jun. 10, 1991) describes the adsorption of volatile substances including halocarbons on various high surface area adsorbents, such as gamma-alumina, activated carbon, high silica zeolite, silica superfine particles, or silica gel, at an adsorption pressure of 1-2 atmospheres, followed by desorption at a reduced pressure, preferably about 1/10 of the pressure at which the adsorption step took place. However, this process is better adapted to halocarbons which are liquids at room temperature, since the system provides liquefied recovered halocarbon. Water is removed using an adsorbent such as potassium Amberlite (K-A) or sodium Amberlite (Na-A) type zeolite, which adsorbs water but does not adsorb organic substances, from the concentrated halocarbon after it is desorbed from the sorbent bed.
Recovery of fluorocarbons from semiconductor etch, CVD, and cleaning process effluents is, moreover, complicated by the presence of other toxic or corrosive components in the gaseous waste (effluent) stream. Typically, the other waste gases can include corrosive species such as HF, or in for example, tungsten etch processes, tunsten oxyfluoride (WOF.sub.4) and tunsten hexafluoride (WF.sub.6). Other species present can include SiF.sub.4, F.sub.2, or COF.sub.2. Plasma processes may generate a large variety of by-product species because of chemical reactions such as rearrangement or scrambling that can occur in the high energy plasma. The table below presents a non-exhaustive list of possible species in the exhaust of a reactor in which C.sub.2 F.sub.6 is used in a plasma process for cleaning steps in silicon processing. Many of these materials are hazardous to personnel because of their corrosivity and toxicity.
In Table I below are listed a number of typical species present in the exhaust stream of a CVD cleaning process.
TABLE I ______________________________________ CVD CHAMBER CLEAN LIST OF POSSIBLE SPECIES IN EXHAUST STREAM Vapor Pressure Boiling Critical Name Formula MW Kpa T .degree. C. T .degree.C. T .degree.C. ______________________________________ Carbonyl COF.sub.2 66.007 5,620 21.1 -84.6 22.8 fluoride Carbon dioxide CO.sub.2 44.011 31.1 Carbon CO 28.010 -191.5 -140.2 monoxide Carbon CF.sub.4 88.005 1.33 -169 -128.0 -45.6 tetrafluoride Decafluoro- C.sub.4 F.sub.10 238.028 330 31.7 -2.0 113.2 butane 1,1-Difluoro- H.sub.2 C.sub.2 F.sub.2 64.035 3,571.5 21.1 -85.7 29.7 ethylene Dioxygen F.sub.2 O.sub.2 70.0 1.33 -120 difluoride Fluorine F.sub.2 37.997 -188.1 -128.8 Hexafluoro Si.sub.2 F.sub.6 170.162 0.0133 -96 disilane Hexafluoro- C.sub.2 F.sub.6 138.012 3,070 21.1 -78.2 19.7 ethane Hexafluoro- C.sub.3 F.sub.6 150.023 687.4 21.1 94.0 propylene Hydrogen HF 20.006 103 20 19.5 188.0 fluoride Octafluoro- C.sub.4 F.sub.8 200.031 274 21.1 -5.8 115.3 cyclobutane Octafluoro- C.sub.3 F.sub.8 188.021 propane Oxygen F.sub.2 O 53.996 0.0133 -205 -144.9 -58.0 difluoride Perfluoro- C.sub.4 F.sub.8 200.031 isobutylene Silicon SiF.sub.4 104.08 0.0133 -155 -14.2 tetrafluoride Tetrafluoro- C.sub.2 F.sub.4 100.016 3,040 21.1 -76.3 33.3 ethylene ______________________________________
The presence of these and other contaminants in process effluents can cause serious problems in adsorption-based recovery/recycle systems. The void space of the adsorbent can be filled with reaction products. This fouling of the adsorbent bed can destroy its ability to be regenerated, which seriously degrades the economics of recovery/recycle. In addition, the feed stream for a recycling process needs to be as clean as possible to make recycle economically attractive. Allowing these corrosive contaminants to remain in the gaseous effluent stream can lead to corrosion of ductwork and valves, and eventually to system failure. And finally, environmental regulations prohibit venting significant quantities of HF or other corrosive gases to the atmosphere.
The presence of these other contaminants can also produce problems in recovery systems using cryogenic recovery systems. Moreover, the presence of water in effluent streams will foul recovery systems using adsorbents as well as cryogenic recovery systems.
In addition to the by-products generated from halocarbon starting materials in the wafer processing step, other hazardous components may be present in the effluent gas stream along with the halocarbon. Corrosive, reactive, flammable and/or poisonous gases may be used in processing steps along with the halocarbon or the effluent from which becomes mixed with the halocarbon-containing effluent gas stream. Examples include arsine (toxic) used in metallorganic chemical vapor deposition, boron trichloride (corrosive) used in etch or cleaning steps and silane (extremely flammable) used in silicide deposition.
Hence, providing a relatively clean, dry, and inert incoming stream to a concentration and recovery unit for halocarbons will help to maximize its capacity and efficiency as well as avoid hazards to personnel and equipment.